The present invention relates to a code error correcting method and apparatuss which allows a picture signal coded in a digital form to be magnetically or optically recorded on a recording medium with an improved efficiency.
In a system for recording a picture signal in the form of a digital signal such as, for example, a digital video tape recorder or VTR, a function or capability of error correction and/or error detection is required for alleviating the influence of erroneous code (also referred to as code error) produced in the course of the recording and/or reproducing process.
Most of the systems for coding the analogue picture signal or image signal, such as component signals including the chrominance (I, Q) signal and the luminance (Y) signal or a composite signal by using eight bits for each of the picture elements or pixels, are usually so arranged as to correct or detect any code error produced at any one of the eight bits. However, in the case of the picture signal, the degree of degradation in the image quality differs significantly in dependence on whether the code error occurs at the first bit position (most significant bit or MSB position) or at the eighth bit (least significant bit or LSB) position. In other words, code error of the more significant bit brings about correspondingly greater degradation in the image or picture quality.
There is disclosed in Heightmann's article "Digital Video Recording: New Results in Channel Coding and Error Protection" contained in "SMPTE Journal", February 1984, p.p. 140-144 an error correction system in which a coding scheme having the highest error correcting capability is adopted for the first bit, and a coding scheme of a slightly reduced error correction capability is adopted for the second bit with a coding scheme of further reduced correction capability being adopted for the third bit, while no error correction is made for the fourth to eighth bits.
The error correction system mentioned above can certainly improve the residual code error ratio (i.e. degradation in the image quality) after the error correction. However, this system suffers a drawback in that because of different coding schemes adopted for the different bits, the structure of all codes to be recorded on the VTR is extremely complicated, whereby the number of units required for realizing this coding system is increased. Besides, some types of the error correction codes are suited for processing on a pixel basis and thus the processings which differ for each of the bits are difficult to be realized by using such type of error code. By the way, there is neither teaching nor suggestion of a solution of the aforenoted problems which contradict one another.
As the error correction code, the b-adjacent code and cyclic redundancy check code are known, as disclosed in Yasuhiro Hirano et al's article "A Study on Variable-Speed Reproduction of the Digital VTR" contained in "SMPTE Journal", June 1983.
A typical example of the hitherto known error correction processings, which are performed on the basis of a pixel represented by one word consisting of a number of bits and in which the b-adjacent code is used as the error correction code will be discussed.
For the picture or pixel signal of eight bits, it is preferred that the parameter b of the b-adjacent code is selected to be equal to 8. By way of example, assuming that the code errors possibly appearing in 18 pixels are to be corrected or detected, the first to eighth bits of the i-th pixel are presented by a.sub.i1, a.sub.i2, a.sub.i3, . . . a.sub.i7, a.sub.i8 and expressed in a matrix W(i), as follows: ##EQU1## Then, matrices P and Q for the correction code each including eight bits are obtained as given by the following expressions: ##EQU2## where T is given by the following matrix. ##EQU3##
The matrices P and W obtained in accordance with the above expressions (2) and (3) for every eighteenth pixel (they may be regarded as codes corresponding to two pixels each consisting of eight bits, respectively) are added upon recording of the coded picture signal on a magnetic tape.
In practice, when the coded picture signal is recorded on the VTR, more than two error correction codes and/or error detection codes are combined for realizing the higher correction and detection capability. In this connection, description will be made by taking as an example a cyclic redundancy check code (hereinafter referred to as CRCC is to in abbreviation). In case CRCC be created for addition to a picture code consisting of eight bits upon recording thereof, the CRCC consisting of eight bits (or an integral multiple of eights bits) is preferred. The principle of creation or generation of the CRCC is known. Describing briefly on the assumption that the CRCC is created from the codes corresponding to eighteen pixels by way of example, 144 bits of W(1) to W(18) are arrayed and dealt with as a single code polynomial Y. Then, a remainder R resulting from division of the polynomial Y by a generating polynomial G represents the code polynomial of the CRCC.
FIGS. 1a and 1b of the accompanying drawings illustrates a code structure in which the aforementioned b-adjacent code and CRCC are arrayed two-dimensionally.
Picture codes are represented by W(1, 1), W(1, 2), W(1, 3) . . . , W(1, 18), W(2, 1), W(2, 2), . . . , W(2, 18), . . . , W(17, 1), W(17, 2), . . . , W(17, 18), W(18, 1), . . . , W(18, 18) in view of the sequence in which these picture codes are generated. From W(i, j) (where i=1, . . . 18), P(j) and Q(j) are created. From W(i, j) (where j=1, . . . , 18), R(i) (where i=1, . . . , 18) is created. From P(j) (where j=1, . . . , 18), R(19) is created. From Q(j) (where j=1, . . . , 18), R(20) is created. Usually, P(j), Q(j) and R(i) are created by utilizing all eight bits constituting each of W(i, j) (where i=1, . . . , 18 and j=1, . . . , 18). Subsequently, W(1, 1), W(1, 2), . . . , W(1, 18), R(1), W(2, 1), W(2, 2), . . . , W(2, 18), R(2), . . . , W(17, 1), W(17, 2), . . . , W(17, 18), R(17), W(18, 1), W(18, 2), . . . , W(18, 18), R(18), P(1), P(2), . . . , P(18), R(19), Q(1), Q(2), . . . , Q(18) and R(20) are recorded on magnetic tape in this order.
The data recorded on the tape is subjected to the code error correction and/or detection with the aid of the known b-adjacent codes and the CRCC. In the case of the numerical example mentioned above, when the presence or inclusion of error bits in the twenty pixels in total inclusive of P and Q is determined by other means, the error detection and correction can be performed if the number of the pixels suffering the bit error is not greater than two, while only the error detection is possible if the error pixels amount to more than three, inclusive. When the probability of pixel containing error is represented by p, the probability q of uncorrectability or incorrigibility is determined as follows: ##EQU4## where p&lt;&lt;1.
This probability q of uncorrectability is too high to be satisfied by those observing the reproduced picture. Accordingly, there is a demand for improvement of the recording medium and the high error correcting capability.